1. Field of the Invention
The invention relates to a magnetic toner or ink and a process for producing the same. More particularly, the invention relates to a magnetic powder with a high degree of black, and good electric and magnetic characteristics which is suitable particularly for a magnetic toner used in electrophotography and a process for producing the magnetic powder.
2. Description of the Prior Arts
One component system developer called a magnetic toner has been known as a developer used in electrophotography. The magnetic toner contains magnetic powder of black color. The use of the black magnetic powder enables one toner to serve as both carrier and toner if the development of a dry type copying machine, thereby to eliminate the need for the carrier in practical use of the developer. Therefore, and operation of a development is easily carried out and accordingly, no control is acquired and an exchange of a carrier is not required and only additional feeding of the toner is required. Moreover, a development unit is simple whereby a labour required for a maintenance is highly reduced and an apparatus is simplified to result in light weight and low cost. Because of those beneficial features, the study of the magnetic toner has been actively conducted recently and some products, developed as a result of the study, have been employed in a commercial scale.
It has been used in the black magnetic powder for the magnetic toner, a magnetite as iron black used for a black pigment which is obtained as a precipitate in a reaction of an aqueous solution (hereinafter referring to as an aqueous solution process). It has been proposed to use various metal oxides, alloys and the like for the black magnetic powder for the magnetic toner. Those materials, when used for it, are attended with many disadvantages. Only the magnetite, therefore, has been practically used eventually. The magnetite powder produced by a wet process using the aqueous solution process has the following various defects or points to be improved, however. When the magnetite is used for the magnetic toner, the toner using it therefore has unsatisfactory characteristics, with the result that one encounters various problems in the use of the toner and meets troubles in a particular copying process because of said advantages.
The magnetite powder produced by the wet process necessarily experiences the aqueous solution process in the course of the production. The magnetic powder thus produced is poor in the heat resistance and the moisture resistance. Usually, the toner is used at about 150.degree. C. At such a temperature, the hue of the powder, the maximum magnetization .sigma. m, the coercive force, the electric resistance, charging amount and the like change, so that the color of the toner and the electric and magnetic characteristics are thermally changed. Further, the magnetite powder has a high hygroscopic property and accordingly, the electrostatic characteristic of the toner is influenced by moisture. In the aqueous solution process, since a large amount of an alkali is used, the residual alkali is contained in the powder even after a washing is carefully performed. The residual alkali considerably deteriorates electrostatic characteristics of the toner resinous component mixed with the residual alkali, adversely changes the quality of the resinous component, or facilitates the aging of the characteristics of the toner. In the wet process, there are many fluctural factors of the process condition for each lot, such as an atmosphere in contact with the solution, an amount of oxygen contained in the solution, the washing conditions, so as to greatly vary the electric and magnetic characteristics, the heat resistance, the moisture resistance, the particle diameter, the particle size distribution and the impurity content. When the powder is used for the magnetic toner, the height of the magnetic brush determined by the magnetic characteristic of the powder, and carrying, the fluidity and the cohesion of the toner vary for each lot. The electrostatic characteristic also varies and hence the picture quality changes. The hue, the heat resistance, the moisture resistance, the compatibility of the powder with the resinous component, a rate of the aging of the resinous component vary, too. Additionally, in the wet process, it is difficult to accurately control the process conditions; the alkali washing is not easy; and a labour is required for a treatment of a waste solution after the washing to increase a cost for the production.
The magnetite produced by the wet process has satisfactory electric and magnetic characteristics and good hue, when it is produced by using much of the labour and under good production. Those characteristics still have some problems to be solved, however. One of those problematic points is to further improve a degree of black. The improvement is desirable, particularly, when it is used for the magnetic toner. Another is to improve the electrostatic characteristic, particularly, the charging amount of the powder. The improvement of this eliminates a variation of the transfer density caused by the resistance variation of a transfer paper which is caused by a moisture variation, and improves the resolution and the graduation, resulting in the improvement of the picture quality. In this respect, it is desired to increase the charging amount of the powder. Still another is to increase a maximum magnetization .sigma. m ranging 50 to 65 emu/g in an external magnetic field of 1000 Oe. With the increase of the maximum magnetization .sigma. m, the height of the magnetic brush is improved. This improvement is desirable.
For overcoming those disadvantages of the magnetite powder for the magnetic toner produced by the conventional wet process, the inventors proposed that the magnetite powder produced by the dry process is more preferable for the magnetic toner than that by the wet process. In the dry process, iron oxide is sintered at 1300.degree.-1500.degree. C. and then, the sintered one is pulverized. The magnetite powder thus produced is satisfactorily stable in the hue, and the electric and magnetic characteristics at the temperature up to about 180.degree. C., good in the heat resistance, small in the humidity absorption, and good in the moisture resistance. With an average particular diameter of less than 1.mu., the paricle size, the particle diameter distribution, and the surface condition of the magnetite powder are stable. The magnetic powder has a good compatibility with a resinous component, and it has high affinity to the resinous component. Further, the magnetic powder is free from such disadvantages as the magnetite obtained by the conventional aqueous solution process which contains an alkaline component remained in the production which causes disadvantageous effects to the resinous component whereby the electrostatics of the magnetic toner are varied. Further, it is free from the disadvantage that there is a variation in the electric and magnetic characteristics, the heat resistance, the moisture resistance, the compatibility of it with the resinous component, and the like.
The magnetite powder prepared by the dry process has the same composition as that of the magnetite powder produced by the wet process. Accordingly, the hue, and the electric and magnetic characteristics are comparable between them. As in the previous case, it is desired to improve the degree of black and, in particular, the charging amount and the maximum magnetization .sigma. m.
The inventors also proposed an excess iron component type ferrite powder having spinel structure, as suitable for the magnetic toner, which comprises components of iron oxide having a ratio of 99.9 to 51 mole % as Fe.sub.2 O.sub.3 and at least one metal oxide selected from the group consisting of manganese oxide, nickel oxide, cobalt oxide, magnesium oxide, copper oxide, zinc oxide, and cadmium oxide at a ratio of 0.1 to 49 mole % as M'O (M' represents Mn, Ni, Co, Mg, Cu, Zn or Cd). The ferrite having the spinel structure is given by EQU (MO).sub.z (FeO).sub.1-z Fe.sub.2 O.sub.3
wherein z is in a range of 0.002 to 0.980 and MO represents one to six kinds of said M'O as one mole. The amount of the oxygen contained is substantially the same as that of the stoichiometric composite. Like the magnetite powder by the dry process, the ferrite powder having the spinel structure is good in the heat resistance, the moisture resistance, and the mixture of it with the resinous component, and does not adversely affect the resinous component. Further, the electric and magnetic characteristics, the heat resistance, the moisture resistance and the mixture of it with the resinous component do not vary for each batch in the production. The electric and magnetic characteristics of the excess iron component type ferrite powder are comparable with those of the magnetite powder. In the group of the ferrite powder, some powders with specific composition has the much better magnetic characteristic, compared to that of the magnetite powder.
The cobalt ferrite and the complex cobalt ferrite in the group of the ferrites have the degree of black as high as that of the magnetite. However, the remaining ferrites are relatively reddish and accordingly, those must be improved in the degree of black. Further for the ferrite having the spinel structure, it is desirable to improve particularly, the maximum magnetization .sigma. m and the charging amount as well so as to improe the spike of the magnetic brush and the picture quality when it is used for the magnetic toner.
The description of the magnetic powder for the magnetic toner having heretofore described may be correspondingly applied to the magnetic powder for the magnetic ink or the ink jet. The improvement of the degree of black and the magnetic characteristic have been accordingly desired in the field of the magnetic ink or the ink jet.